1. Field of the Invention
The invention is directed to a method for synchronizing data streams having a data stream comprising a plurality of data units in which a data stream is split into two congruent, redundantly fashioned data sub-streams.
2. Description of the Related Art
In order to be able to transmit information such as voice information or data via communication networks, the information to be transmitted are combined to form data units. In contemporary communication networks, these data units are fashioned in packets or cells such as ATM cells, but care must be taken to preserve the sequence of the data units in the transmission. When the sequence of the data units is not directly assured, for example by not using transmission sequence numbers, serious errors can occur in the communication of the data units. Further errors and losses can occur in that individual data units are either disturbed (modified content) or entirely missing (for example, due to buffer overflows in switching equipment).
In the Prior Art, the dependability of connections in communication networks is enhanced by redundant transmission via two physically separate paths, however, this scheme has problems in the synchronization of the redundant data streams at the receiver and the switching between the data streams.
The publication, Ohta, H., Ueda, E., “Hitless Line Protection Switching Method for ATM Networks”, International Conference on Communications, Proceedings ICC '93, pp. 272-276, 1993, discloses a method with which the payload data transported in ATM cells are employed for synchronization.
This known method is specifically disclosed for ATM cells. It comprises a total of three operating conditions (search process, confirmation process, out-of-alignment process). In the “search process” condition, an ATM cell of an arbitrarily selected data stream is stored and compared to all following ATM cells of the other data stream. When two ATM cells agree with one another, then the measured delay is assumed as a running time difference between the signals. When no matching ATM cell for the intermediately stored ATM cell was capable of being found beyond a maximum length, then another ATM cell of the other data stream is intermediately stored and compared.
Since the content of different ATM cells need not necessarily be different, this assumption that identical ATM cells in the two data streams must correspond to one another is not always correct, and hence, the assumption is checked by multiple repetition, where an ATM cell is respectively stored and compared to the ATM cell of the other data stream that corresponds to the previously identified delay. Only then is the next ATM cell stored and compared. But this makes the method become very slow.
The “confirmation process” condition follows when the search process was successfully run. In this condition, a delay having the size of the measured running time difference is inserted into the leading data stream (that from which the ATM cell was respectively stored). Subsequently, the delayed ATM cells of the leading data stream are compared to the current ATM cells of the trailing data stream. When this comparison is also repeatedly correct, then the synchronism is considered detected and a switch into the “out-of-alignment detection process” condition ensues.
In the “out-of-alignment detection process”, the delayed ATM cells of the leading data stream continue to be compared to the current ATM cells of the trailing data stream. When more than a predetermined plurality of comparisons are successively false, then a re-synchronization is triggered.
The prior art discloses the possibility of switching from one data stream to the other at any time requires that an ATM cell can only be forwarded when it was received from both data streams. This method, however, means that a delay amounting to the maximum of the two data streams is always inserted, and results in a very long duration until the synchronism of two data streams has been detected. Moreover, this method has no tolerance with respect to the lack of individual ATM cells, so that every omission of an ATM cell from a data stream triggers a new synchronization process.